V/+BRR1

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V/+BRR1

This website contains an archive of files for the Acorn Electron, BBC Micro, Acorn Archimedes, Commodore 16 and Commodore 64 computers, which Dominic Ford has rescued from his private collection of floppy disks and cassettes.

Some of these files were originally commercial releases in the 1980s and 1990s, but they are now widely available online. I assume that copyright over them is no longer being asserted. If you own the copyright and would like files to be removed, please contact me.

Tape/disk:	Home » Personal collection » Acorn ADFS disks » Electron_User_Group » EUG_55.ADF
Filename:	V/+BRR1
Read OK:	✔
File size:	147F bytes
Load address:	56204556
Exec address:	52422B2E

Duplicates

There are 3 duplicate copies of this file in the archive:

AEW website » eug » eug_3_5_discs_Eug-55_A-EUG55.adf » V/+BRR1
AEW website » eug » eug_5_25_discs_Eug-55_D-EUG55.dsd » V.+BRR1
Personal collection » Acorn ADFS disks » Electron_User_Group » EUG_55.ADF » V/+BRR1
Personal collection » Acorn ADFS disks » Greaseweazled » adfs_EUG_55.adf » V/+BRR1

File contents

                          BASIC ROM ROUTINES
                         Christopher Dewhurst

THE Basic Rom is that dark and mysterious area of memory that lies 
beyond the screen, starting at &8000 and stretching up to &BFFF. It's
the backstage department containing the machine code needed to interpret
your Basic commands. But have you ever wondered if we can use some of 
that machine code in our own programs? Have you, for instance, struggled
to write an assembler routine to print a number on the screen, when one
must surely exist somewhere in the Basic Rom? Well, wonder no more, 
because there is indeed such a routine, and in this article we'll be 
exploring that and a lot more besides.
   Before we go any further, however, a word of caution. The best 
machine code is specific machine code written for a specific job; Basic
Rom routines are general-purpose routines, and are not the answer to
everything. Having said that, if speed is not your main priority, then
the Rom routines are ideal. They make your programs smaller and smarter,
provided you use them properly - and this usually involves some fairly
tricky setting up - so listen carefully.
   I learnt a lot about the Basic Rom by exploring around it and experi-
menting with it myself. I also picked up a few tips from THE ADVANCED
BASIC ROM USER GUIDE by Colin Pharo (Cambridge Micro Centre, 1984). 
Roland Waddilove also presented a series of excellent articles on the 
subject in ELECTRON USER; if you still have these paper beauties, dig 
out the November 1988 issue for a rundown on mathematical Rom routines. 
However, I will be concentrating on routines which print numbers in hex 
or decimal, the random number generator, and printing strings of text.
   In case you're wondering, BBC Master owners won't be left out of the
discussion this time. I have done quite a bit of disassembling of the
Basic 4 Rom to find out where equivalent routines to Basic 2 reside. 
Basic 2 is the Rom fitted in the BBC B and Electron, and Basic 4 is the 
one fitted in the Master (Like the Plus 2, for some reason Basic 3
never was). When I talk about a Rom routine, I will specify both the
Basic 2 and Basic 4 addresses - together with examples and commentaries
on how to use them - so it is up to you to use the correct one depending
on which computer you have. If you experience any difficulties - or if 
you have additional hints and tips - just write in.
   Right, down to business. We must first get to know what is called the
Integer Work Area, or IWA for short. This is just a sequence of four
bytes in zero page, located at &2A-&2D. Before Basic can work on an
integer variable, be it adding a number to it or printing it out, it 
must be put into the IWA. Fortunately, life is made easier with the help
of a couple of routines which copy an integer variable, either from zero
page or from the main memory, to the IWA:

1.      Routine: Copy 4-byte integer from zero page to the IWA
        Basic 2 address: &AF56
        Basic 4 address: &AA80

        Entry: X = zero page offset at which the integer to be copied is
        located.
        Exit: IWA contains the integer.
        Ex.:    LDX #&70  \integer at &70-3
                JSR &AF56  \copy to IWA

2.      Routine: Copy 4-byte integer from memory to the IWA
        Basic 2: &B336
        Basic 4: &B1AA

        Entry: &2A/&2B contain address of the integer.
        Exit: IWA contains the integer.
        Ex.:    LDA #integer MOD 256
                STA &2A
                LDA #integer DIV 256
                STA &2B	
                JSR &B336
                ...
                .integer EQUD &12345678

There are also two routines which do the opposite of above. The one at 
&BE44 (Basic 2)/&BDC6 (Basic 4) copies the IWA to a zero-page location, 
X being set to the zero page location on entry. The routine at &B4C6 
(Basic 2)/&B347 (Basic 4) copies the IWA to a location in main memory 
whose address is held in &37/&38.

3.      Routine: Print a string
        Basic 2: &BFCF
        Basic 4: &BECF

        Entry: The string must follow the JSR &BFCF instruction, and be
        terminated by a byte of value &80 or greater.
        Ex.:    JSR &BFCF
                EQUS "Hello there.":NOP
                ...

Notice how I've used the NOP instruction to terminate the string. The
NOP opcode has a value of &EA, which satisfies the condition of being
&80 or greater. The important point to remember is that program
execution continues AFTER that NOP instruction. In machine code, every
time a JSR instruction is executed the current address is pushed onto
the stack. Basic pulls this address from the stack, stores it in zero
page locations &37/&38 and uses indirect addressing to get the bytes of
the string. By the time the string has been printed, &37/&38 contains
the address of the next instruction after the string in the program
that called the routine. The disadvantage of this routine, however, is
that while you can include control codes (to turn off the cursor for
instance) you can't print out a string of graphics characters because
they have an ASCII value of &80 or above, which, as we said, is used to
terminate the string.

This article continued in V.+BRR2. First published EUG #55.

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00000010  20 20 20 20 20 20 20 20  20 20 42 41 53 49 43 20  |          BASIC |
00000020  52 4f 4d 20 52 4f 55 54  49 4e 45 53 0d 20 20 20  |ROM ROUTINES.   |
00000030  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 20  |                |
00000040  20 20 20 20 20 20 43 68  72 69 73 74 6f 70 68 65  |      Christophe|
00000050  72 20 44 65 77 68 75 72  73 74 0d 0d 54 48 45 20  |r Dewhurst..THE |
00000060  42 61 73 69 63 20 52 6f  6d 20 69 73 20 74 68 61  |Basic Rom is tha|
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00000080  72 69 6f 75 73 20 61 72  65 61 20 6f 66 20 6d 65  |rious area of me|
00000090  6d 6f 72 79 20 74 68 61  74 20 6c 69 65 73 20 0d  |mory that lies .|
000000a0  62 65 79 6f 6e 64 20 74  68 65 20 73 63 72 65 65  |beyond the scree|
000000b0  6e 2c 20 73 74 61 72 74  69 6e 67 20 61 74 20 26  |n, starting at &|
000000c0  38 30 30 30 20 61 6e 64  20 73 74 72 65 74 63 68  |8000 and stretch|
000000d0  69 6e 67 20 75 70 20 74  6f 20 26 42 46 46 46 2e  |ing up to &BFFF.|
000000e0  20 49 74 27 73 0d 74 68  65 20 62 61 63 6b 73 74  | It's.the backst|
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000001e0  70 72 69 6e 74 20 61 20  6e 75 6d 62 65 72 20 6f  |print a number o|
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000002e0  20 77 6f 72 64 20 6f 66  20 63 61 75 74 69 6f 6e  | word of caution|
000002f0  2e 20 54 68 65 20 62 65  73 74 20 0d 6d 61 63 68  |. The best .mach|
00000300  69 6e 65 20 63 6f 64 65  20 69 73 20 73 70 65 63  |ine code is spec|
00000310  69 66 69 63 20 6d 61 63  68 69 6e 65 20 63 6f 64  |ific machine cod|
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00000340  73 69 63 0d 52 6f 6d 20  72 6f 75 74 69 6e 65 73  |sic.Rom routines|
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00000390  69 6e 67 2e 20 48 61 76  69 6e 67 20 73 61 69 64  |ing. Having said|
000003a0  20 74 68 61 74 2c 20 69  66 20 73 70 65 65 64 20  | that, if speed |
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000003d0  74 68 65 20 52 6f 6d 20  72 6f 75 74 69 6e 65 73  |the Rom routines|
000003e0  20 61 72 65 20 69 64 65  61 6c 2e 20 54 68 65 79  | are ideal. They|
000003f0  20 6d 61 6b 65 20 79 6f  75 72 20 70 72 6f 67 72  | make your progr|
00000400  61 6d 73 20 73 6d 61 6c  6c 65 72 20 61 6e 64 20  |ams smaller and |
00000410  73 6d 61 72 74 65 72 2c  0d 70 72 6f 76 69 64 65  |smarter,.provide|
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00000440  69 73 20 75 73 75 61 6c  6c 79 20 69 6e 76 6f 6c  |is usually invol|
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00000460  74 72 69 63 6b 79 20 73  65 74 74 69 6e 67 20 75  |tricky setting u|
00000470  70 20 2d 20 73 6f 20 6c  69 73 74 65 6e 20 63 61  |p - so listen ca|
00000480  72 65 66 75 6c 6c 79 2e  0d 20 20 20 49 20 6c 65  |refully..   I le|
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000004e0  74 20 6d 79 73 65 6c 66  2e 20 49 20 61 6c 73 6f  |t myself. I also|
000004f0  20 70 69 63 6b 65 64 20  75 70 20 61 20 66 65 77  | picked up a few|
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00000510  44 56 41 4e 43 45 44 0d  42 41 53 49 43 20 52 4f  |DVANCED.BASIC RO|
00000520  4d 20 55 53 45 52 20 47  55 49 44 45 20 62 79 20  |M USER GUIDE by |
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000005b0  4c 45 43 54 52 4f 4e 20  55 53 45 52 3b 20 69 66  |LECTRON USER; if|
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00000810  73 6f 6d 65 20 72 65 61  73 6f 6e 20 42 61 73 69  |some reason Basi|
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00000850  20 49 20 77 69 6c 6c 20  73 70 65 63 69 66 79 20  | I will specify |
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00000870  20 61 6e 64 20 42 61 73  69 63 20 34 20 61 64 64  | and Basic 4 add|
00000880  72 65 73 73 65 73 20 2d  20 74 6f 67 65 74 68 65  |resses - togethe|
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000008c0  68 65 6d 20 2d 20 73 6f  20 69 74 20 69 73 20 75  |hem - so it is u|
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00000980  64 6f 77 6e 20 74 6f 20  62 75 73 69 6e 65 73 73  |down to business|
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00000ac0  65 20 65 61 73 69 65 72  20 77 69 74 68 20 74 68  |e easier with th|
00000ad0  65 20 68 65 6c 70 0d 6f  66 20 61 20 63 6f 75 70  |e help.of a coup|
00000ae0  6c 65 20 6f 66 20 72 6f  75 74 69 6e 65 73 20 77  |le of routines w|
00000af0  68 69 63 68 20 63 6f 70  79 20 61 6e 20 69 6e 74  |hich copy an int|
00000b00  65 67 65 72 20 76 61 72  69 61 62 6c 65 2c 20 65  |eger variable, e|
00000b10  69 74 68 65 72 20 66 72  6f 6d 20 7a 65 72 6f 0d  |ither from zero.|
00000b20  70 61 67 65 20 6f 72 20  66 72 6f 6d 20 74 68 65  |page or from the|
00000b30  20 6d 61 69 6e 20 6d 65  6d 6f 72 79 2c 20 74 6f  | main memory, to|
00000b40  20 74 68 65 20 49 57 41  3a 0d 0d 31 2e 20 20 20  | the IWA:..1.   |
00000b50  20 20 20 52 6f 75 74 69  6e 65 3a 20 43 6f 70 79  |   Routine: Copy|
00000b60  20 34 2d 62 79 74 65 20  69 6e 74 65 67 65 72 20  | 4-byte integer |
00000b70  66 72 6f 6d 20 7a 65 72  6f 20 70 61 67 65 20 74  |from zero page t|
00000b80  6f 20 74 68 65 20 49 57  41 0d 20 20 20 20 20 20  |o the IWA.      |
00000b90  20 20 42 61 73 69 63 20  32 20 61 64 64 72 65 73  |  Basic 2 addres|
00000ba0  73 3a 20 26 41 46 35 36  0d 20 20 20 20 20 20 20  |s: &AF56.       |
00000bb0  20 42 61 73 69 63 20 34  20 61 64 64 72 65 73 73  | Basic 4 address|
00000bc0  3a 20 26 41 41 38 30 0d  0d 20 20 20 20 20 20 20  |: &AA80..       |
00000bd0  20 45 6e 74 72 79 3a 20  58 20 3d 20 7a 65 72 6f  | Entry: X = zero|
00000be0  20 70 61 67 65 20 6f 66  66 73 65 74 20 61 74 20  | page offset at |
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00000c50  20 20 20 45 78 2e 3a 20  20 20 20 4c 44 58 20 23  |   Ex.:    LDX #|
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00000c70  20 26 37 30 2d 33 0d 20  20 20 20 20 20 20 20 20  | &70-3.         |
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00000e00  20 20 20 20 20 20 20 20  20 20 20 20 20 20 20 2e  |               .|
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00001400  26 38 30 20 6f 72 20 61  62 6f 76 65 2c 20 77 68  |&80 or above, wh|
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00001420  20 69 73 20 75 73 65 64  20 74 6f 0d 74 65 72 6d  | is used to.term|
00001430  69 6e 61 74 65 20 74 68  65 20 73 74 72 69 6e 67  |inate the string|
00001440  2e 0d 0d 54 68 69 73 20  61 72 74 69 63 6c 65 20  |...This article |
00001450  63 6f 6e 74 69 6e 75 65  64 20 69 6e 20 56 2e 2b  |continued in V.+|
00001460  42 52 52 32 2e 20 46 69  72 73 74 20 70 75 62 6c  |BRR2. First publ|
00001470  69 73 68 65 64 20 45 55  47 20 23 35 35 2e 0d     |ished EUG #55..|
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